MPC5xxx: Change names of defines related to IPB and PCI clocks.

Both CFG_PCISPEED_66 and CFG_IPBSPEED_133 are misnamed, as defining
them does not cause PCI or IPB clocks to run at the specified speed.
Instead, they configure divisors used to calculate said clocks. This
patch renames the defines according to their real function.

Signed-off-by: Grzegorz Bernacki <gjb@semihalf.com>
Acked-by: Bartlomiej Sieka <tur@semihalf.com>